Heat resistant pigments are required for coloring synthetic materials, in the paper industry and in coil coat systems in the lacquer industry. The yellow pigments used are mainly lead chromates and cadmium sulphides but compounds containing bismuth, vanadium and molybdenum are also used. One disadvantage of these heat resistant yellow pigments is their high cost. There is, therefore, a demand for replacing them by less expensive pigments.
Iron oxide yellow pigments which have an intense color and are chemically resistant and highly weather resistant would be possible substitutes but the usual iron oxide yellow pigments are only of limited use as heat resistant yellow pigments since they change their color to reddish brown at temperatures above 200.degree. C. due to decomposition to .alpha.-Fe.sub.2 O.sub.3.
If they are to be widely used as heat resistant yellow pigments, they should be suitable for use not only in the paper industry and in coil coat systems but especially also as coloring agents in thermoplastic synthetic materials which are processed at high temperatures.
Table 1 gives the usual processing temperatures employed for thermoplastic synthetic materials.
TABLE 1 ______________________________________ Processing temperatures of thermoplastic synthetic materials Synthetic material Processing Temperature ______________________________________ Polyvinyl chloride from 160.degree. C. HD-polyethylene from 240.degree. C. Polypropylene from 260.degree. C. Styrene polymer from 220.degree. C. Polyamide from 230.degree. C. Polycarbonates from 300.degree. C. ______________________________________
It has long been attempted to improve the heat resistance of iron oxide yellow by coating the pigments. GERMAN Pat. No. 2,740,861 and U.S. Pat. No. 4,053,325 describe the coating of iron oxide pigments with up to 20% by weight of a metal metaphospate having the composition Al(PO.sub.3).sub.3, Fe(PO.sub.3).sub.3, Ca(PO.sub.3).sub.2, Ba(PO.sub.3).sub.2 but the heat resistance achieved is only up to a temperature of about 230.degree. C.
Other processes for coating iron oxide yellow serve to provide protection against sintering during the subsequent dehydration to .alpha.-Fe.sub.2 O.sub.3, reduction to Fe.sub.3 O.sub.4 and oxidation to .gamma.-Fe.sub.2 O.sub.3. The starting material used for these processes is needle shaped .alpha.-FeOOH in which processes the needle shape is required to be preserved over several reaction stages up to needle shaped Fe.sub.3 O.sub.4 or .gamma.-Fe.sub.2 O.sub.3 and protected against sintering.
The after treatments proposed, namely the application of up to 5% by weight of silicone oil, 2% by weight of Fe, Cr, Mn, Al, Ga, In, La, Ce, rare earths (GERMAN Pat. No. 2,339,142) or 0.1 to 5% by weight of oxides or phosphates of Zn, Al, Ce, Cr, Fe, Mn, Sb, Sn, Ti, Zr (GERMAN Pat. No. 2,507,420) and 0.1 to 6% by weight of phosphate (GERMAN Pat. No. 1,592,214) must not prevent the dehydration of .alpha.-FeOOH to .alpha.-Fe.sub.2 O.sub.3 but should serve solely as protection against sintering. These coating processes are unsuitable for the preparation to sufficiently heat resistant yellow pigments. Their starting materials are preliminary products for the preparation of magnetic pigments and are coloristically unsuitable for use as yellow pigments.
Since no iron oxide yellow pigments having sufficient heat resistance for general use have hitherto been obtained by conventional methods, it has repeatedly been proposed (U.S. Pat. No. 4,376,677, JAPAN Pat. No. 53,102,298, U.S. Pat. No. 4,376,656) to produce heat resistant iron oxide yellow pigments by hydrothermal processes.
The hydrothermal processes proposed, which in the case of U.S. Pat. Nos. 4,376,677 and 4,376,656 comprise two successive hydrothermal syntheses, are uneconomical.
They partly lead to pigments containing only a minor proportion of iron ions (U.S. Pat. No. 4,376,677, example 1; about 66 mol % Al, 33 mol % Fe, 1 Mol % si) and, therefore, only have a weak color.
The present invention therefore relates to the preparation of an iron oxide yellow pigment with improved heat resistance which does not have the disadvantages of the described pigments belonging to the state of the art.